Any turbine or turbine section which is exposed to a throughflow of a working medium in the form of steam is understood by a steam turbine in the sense of the present application. In contrast to this, gas turbines are exposed to a throughflow of gas and/or air as working medium which, however, is subject to totally different temperature and pressure conditions than steam in the case of a steam turbine. Unlike gas turbines, in steam turbines the working medium which flows into a turbine section at the highest temperature has at the same time the highest pressure, for example. An open cooling system, which is open to the flow passage, can be realized in gas turbines even without external feed of cooling medium to turbine sections. For a steam turbine, an external feed of cooling medium ought to be provided. Gas turbines which relate to the prior art cannot even be consulted for assessment of the present application subject matter for this reason.
A steam turbine customarily comprises a rotatably supported rotor which is fitted with blades and arranged inside a casing or casing shell. When the interior space of the flow passage which is formed by the casing shell is exposed to a throughflow of heated and pressurized steam, the rotor, via the blades, is made to rotate by means of the steam. The blades of the rotor are also referred to as rotor blades. Customarily suspended on the inner casing, moreover, are stationary stator blades which along an axial extension of the body engage in the interspaces of the rotor blades. A stator blade is customarily retained at a first point along an inner side of the steam turbine casing. In this case, it is customarily part of a stator blade row which comprises a number of stator blades which are arranged along an inside circumference on an inner side of the steam turbine casing. In this case, each stator blade points radially inward by its blade airfoil. A stator blade row on the mentioned first point along the axial extension is also referred to as a stator blade cascade or ring. Customarily, a number of stator blade rows are connected in series. A further second blading is correspondingly retained along the inner side of the steam turbine casing at a second point along the axial extent downstream of the first point. A pair comprising a stator blade row and a rotor blade row is also referred to as a blading stage.
The casing shell of such a steam turbine can be formed from a number of casing segments. The stationary casing component of a steam turbine or of a turbine section which along the longitudinal direction of the steam turbine has an interior space in the form of a flow passage which is provided for the throughflow by the working medium in the form of steam is especially to be understood by the casing shell of the steam turbine. This can be an inner casing and/or a stator blade carrier, depending on steam turbine type. However, provision can also be made for a turbine casing which does not have an inner casing or stator blade carrier.
For efficiency reasons, the design of such a steam turbine may be desirable for so-called “high steam parameters”, therefore especially high steam pressures and/or high steam temperatures. However, for material engineering reasons a temperature increase is especially not possible without limitation. In order to also enable a reliable operation of the steam turbine at particularly high temperatures in this case a cooling of individual parts or components may therefore be desirable. Without efficient cooling, significantly more expensive materials (e.g. nickel-based alloys) would be required in the case of increasing temperatures.
In the case of the previously known cooling methods, especially for a steam turbine body in the form of a steam turbine casing or of a rotor, a differentiation is to be made between an active cooling system and a passive cooling system. In the case of an active cooling system, cooling by means of a cooling medium which is fed separately to the steam turbine body, i.e. in addition to the working medium, is put into effect. On the other hand, passive cooling is carried out purely by a suitable conduction or use of the working medium. Up to now, steam turbine bodies have been preferably passively cooled.
All cooling methods which are known to date for a steam turbine casing, providing they chiefly concern active cooling methods, therefore provide at best a directed inflow to a separate turbine section which is to be cooled and are restricted to the inflow region of the working medium, at any event including the first stator blade ring. In the case of loading of conventional steam turbines with higher steam parameters, this can lead to an increased thermal loading which acts upon the entire turbine and which could be only inadequately reduced by means of conventional cooling of the casing which is described above.
Embodiments of steam turbines are known which in addition to a first flow passage have a second flow passage, wherein both the first flow passage and the second flow passage are arranged inside a casing. Such constructional forms are also referred to as compact turbines. Embodiments are known in which the first flow passage is designed for high-pressure blading and the second flow passage is designed for intermediate-pressure blading. The flow directions of the first flow passage and of the second flow passage point in this case in opposite directions in order to minimize the thrust compensation as a result. In the main, such constructional forms comprise a rotor which is designed with a high-pressure region and an intermediate-pressure region and is rotatably supported inside an inner casing, wherein an outer casing is arranged around the inner casing. The high-pressure region is designed for live steam temperatures. After the live steam has flowed through the high-pressure region, the steam flows to a reheater and is brought to a higher temperature there, and then flows through the intermediate-pressure region of the steam turbine.
The limits of use of such rotors are defined by thermally highly stressed regions. With temperatures becoming greater, the essential strength characteristic value decreases superproportionally. As a result, maximum permissible shaft diameters ensue which especially lead to limitations in 60 Hertz applications, which concerns the rotor-dynamic degree of slenderness of the rotor. Therefore, upon reaching limits of use, in the case of a monoblock rotor a change is usually made to the next best material which withstands the thermal demands or a rotor is of a welded construction, wherein two materials are designed in each case for the thermal stresses.
It would be desirable to have an effective cooling system in a steam turbine component, especially for a steam turbine operated at high temperature.